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eHam Forums => Antennas and Towers and more => Topic started by: HS0ZIB on April 16, 2011, 09:40:13 PM



Title: How to estimate the 'footprint' for guying a tower
Post by: HS0ZIB on April 16, 2011, 09:40:13 PM
Is there a quick and easy formula to calculate the distance/location of guying wires for an antenna tower of defined height?  (Presumably there is an optimum angle between the tower and the guying...)

I want to do a back-of-postcard calculation so that I know the required dimensions of land plots that are available for purchase in my locality.

Thanks

Simon


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: K9KJM on April 16, 2011, 09:53:57 PM
Basic rule of thumb is the guy anchors enter the earth at 80% of the tower height.

(A 100 foot tall tower would have the guy anchors 80 feet away from the tower base.)


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: WB2WIK on April 17, 2011, 05:21:17 PM
Basic rule of thumb is the guy anchors enter the earth at 80% of the tower height.

(A 100 foot tall tower would have the guy anchors 80 feet away from the tower base.)

I agree, that's about right.  I've used 70% and even 90%+ on occasion when it was convenient, but 80% is a good rule of thumb.


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: KB9CRY on April 17, 2011, 07:22:01 PM
Is there a quick and easy formula to calculate the distance/location of guying wires for an antenna tower of defined height?  (Presumably there is an optimum angle between the tower and the guying...)

I want to do a back-of-postcard calculation so that I know the required dimensions of land plots that are available for purchase in my locality.

Thanks

Simon

a2 + b2 = c2

a squared plus b squared equals c squared.

Ask any 6 year old.


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: HS0ZIB on April 18, 2011, 06:24:43 AM
Quote
a2 + b2 = c2
a squared plus b squared equals c squared.
Ask any 6 year old.

Yes, I am more than 6 years old and I know how to do squares and square roots.  But that wasn't what I asked.

Happily K9KJM provided the useful details that I needed

Simon


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: W4VR on April 18, 2011, 08:02:48 AM
When I was in the broadcast consulting business many years ago, if a client would ask how much land he needed for a tower we would tell him to use 70% guying.  In other words the anchor points would be 210 feet away from the tower base on a 300 foot tower.  This translates into a requirement for 4 acres of property to accomodate that tower.


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: HS0ZIB on April 18, 2011, 04:44:33 PM
@W4VR, thanks for also confirming this 70-80% rule.

In any case, it also confirms that the land plot I'm looking at is too small for a guyed mast, so I'm going to have to consider a free-standing tower

Simon


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: K4KYV on April 19, 2011, 08:15:26 PM
The Rohn tower book says 80%.


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: K9KJM on April 19, 2011, 09:56:11 PM
The standard 80% rule is for a tower guyed every 120 degrees, Three sets of guys.
You can get by with lots less real estate by guying every 90 degrees, (Four sets of guy wires.)



Title: RE: How to estimate the 'footprint' for guying a tower
Post by: AD4U on April 20, 2011, 05:41:09 AM
Google "Rohn Towers".  They give more info on this site than you could ever use.

BTW - the 80% rule is a good one.

Dick  AD4U


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: K4HYZ on April 20, 2011, 10:37:33 PM
If you need to save space?    You can make your guy anchors come up out of the ground 6-10'.  I have never seen this in the states but here in thailand i see it all the time...


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: W6RMK on April 21, 2011, 07:29:44 AM
Raising the guy anchors is a way to "cut off" the vertex of the triangle, but does significantly change the engineering.  You're basically building a little tower to hold the top of the guy, so it needs to be designed to not bend, pull out of the ground, etc.

Say you do something like use a piece of (fairly large) pipe as the elevated guy anchor. A lot depends on what is "good enough" from a failure standpoint.  If you're willing to tolerate your elevated guy anchor bending over when the big tower sees a big wind load (letting the big tower lean somewhat), it's a lot easier.  (that's sort of a fail-soft mechanism.. the whole thing doesn't come crashing to the ground, but you've also broken something that needs to be fixed)
(BTW, filling the pipe with concrete doesn't change things very much, apparently)

If you do make the elevated guy post strong enough to not bend under the load, then the foundation/base for that post has to be remarkably large to prevent it from "levering" out of the ground.   If you can arrange to have a long skinny hole drilled to be filled with a rebar cage and your post, that works pretty well.  If you're depending just on the mass of the concrete (as opposed to the resisting force of the surrounding soil) it takes a lot more.

there's been some discussion on the TowerTalk reflector (contesting.com) over the years about elevated guys, with some sample engineering discussed.  Might be worth a search of the archives.


Title: RE: How to estimate the 'footprint' for guying a tower
Post by: W4VR on April 22, 2011, 02:48:09 PM
Most manufacturers suggest a standard outer guy radius of 70 percent (80 percent for microwave) of the overall tower height. At this radius, downloads on the tower structure from the guys are minimized resulting in the least-cost tower to meet a particular antenna and wind load requirement. As the guy radius is reduced below 70 percent, the downward pulling force of the guy system is increased as a result of the sharper angle of the guy relative to the structure. This increased downward force will result in the need for a stronger, more expensive tower and a heavier, more expensive guy system. If properly designed, shortening the guy system to 40-50 percent of tower height does not compromise the tower's ability to support the load intended or to meet the requirements of the EIA RS-222 code. However, tower and foundation costs will definitely be greater.